path: improve documentation for Dir

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6495059

src/pkg/path/path.go

@@ -198,7 +198,8 @@ func IsAbs(path string) bool {
 }
 
 // Dir returns all but the last element of path, typically the path's directory.
-// The path is Cleaned and trailing slashes are removed before processing.
+// After dropping the final element using Split, the path is Cleaned and trailing
+// slashes are removed.
 // If the path is empty, Dir returns ".".
 // If the path consists entirely of slashes followed by non-slash bytes, Dir
 // returns a single slash. In any other case, the returned path does not end in a

src/pkg/path/path_test.go

@@ -200,6 +200,7 @@ var dirtests = []PathTest{
 	{"x/", "x"},
 	{"abc", "."},
 	{"abc/def", "abc"},
+	{"abc////def", "abc"},
 	{"a/b/.x", "a/b"},
 	{"a/b/c.", "a/b"},
 	{"a/b/c.x", "a/b"},